LOCALIZATION OF D(2) DOPAMINE-RECEPTORS IN VERTEBRATE RETINAE WITH ANTIPEPTIDE ANTIBODIES

Dopamine plays an important role in modulating various aspects of reti nal signal processing. The morphology of dopaminergic neurons and its physiological effects are well characterized. Two classes of receptor molecules (D1 and D2) were shown pharmacologically to mediate specific actions, with differences between individual groups of vertebrates. I n an attempt to better understand dopaminergic mechanisms at the cellu lar level, we used antisera against D2 receptors and investigated the localization of the dopamine D2 receptor in the retinae of rat, rabbit , cow, chick, turtle, frog, and two fish species with immunofluorescen ce techniques. Antisera were raised in rabbits to two oligopeptides pr edicted from rat D2 receptor cDNA; one specific for the splice-variant insertion in the third cytoplasmic loop and the other directed toward s the extracellular amino terminal region shared by both short and lon g isoforms. Preadsorption with the synthetic peptide resulted in a sig nificant reduction of label, indicating the presence of specific bindi ng in all species except turtle and goldfish. The pattern of labelling produced by the two antisera was essentially identical; however, the staining obtained with antiserum to the extracellular motif was always more intense. Specific staining was present in photoreceptor inner an d outer segments, and in the outer and inner plexiform layers of all s pecies. In mammals and chick, strongly fluorescent perikarya were obse rved in the ganglion cell layer and at the proximal margin of the inne r nuclear layer. Label may be present in the pigment epithelium but co uld not be established beyond doubt. This pattern of labelling is in a ccordance with previous observations on D2 receptor localization by me ans of radioactive ligand binding and in situ hybridization techniques . It suggests that retinal dopamine acts as a neuromodulator as well a s a transmitter. In the distal retina, it may reach its targets via di ffusion over considerable distances, even crossing the outer limiting membrane; in the inner and outer plexiform layers, conventional synapt ic transmission seems to coexist with paracrine addressing of more dis tant targets, and D2 receptors are expressed by both amacrine and gang lion cells.